The envelope glycoprotein (env) of human immunodeficiency virus (HIV) plays a central role in the pathogenicity of AIDS but the arrangement of subunits in this protein is still unknown. Scanning transmission electron microscopy (STEM) has been applied to determine the subunit arrangement of purified, cloned derivatives of the envelope protein gp140 in both HIV and SIV. Macromolecular complexes were adsorbed onto thin carbon supports and were plunge-frozen, freeze-dried and imaged in dark-field STEM at low electron dose. Images were analyzed quantitatively using a calibration standard of tobacco mosaic virus to determine the distributions of molecular weights and hence the oligomeric states of the glycoproteins. Results are compared with sedimentation data obtained from analytical ultracentrifugation.